The boom of concrete distributing device usually comprises 3-6 knuckle arms, each of which is controlled by an actuating mechanism to stretch out or fold back.
Presently, hydraulic systems for boom control of concrete distributing devices are usually single loop hydraulic systems that employ a single pump and a plurality of actuating mechanisms.
To ensure the plurality of actuating mechanisms can act simultaneously and the movement speeds of the actuating mechanisms can be regulated steplessly independent of the load, the hydraulic system is usually a load-sensitive hydraulic system with pressure compensation function. Hereunder a hydraulic system with two actuating mechanisms will be described as an example.
FIG. 1 shows a hydraulic system with two actuating mechanisms. The following functional relation exists in the hydraulic system shown in FIG. 1:
                    Δ        ⁢                                  ⁢                  P          1                    =                                    P            1            ′                    -                      P            1                          =                              F            1                    /                      S            1                                ;                  Δ        ⁢                                  ⁢                  P          2                    =                                    P            2            ′                    -                      P            2                          =                              F            2                    /                      S            2                                                  Q        1            =              λ        ⁢                                  ⁢                  A          1                ⁢                                            2              ⁢              Δ              ⁢                                                          ⁢                              P                1                                      ρ                                ;                  Q        2            =              λ        ⁢                                  ⁢                  A          2                ⁢                                            2              ⁢              Δ              ⁢                                                          ⁢                              P                2                                      ρ                              
Once the pressure of reducing valve 1 and reducing valve 2 is set, the flow through the actuating mechanism 1 and actuating mechanism 2 (i.e., the movement speed of the actuating mechanisms) will only depend on the opening area A1 and A2 of the electrical proportional control valves, and will be irrelevant to the load, i.e.:Q1=f(A1); Q2=f(A2)
FIG. 2 is a structural block diagram of boom control system on most of the existing concrete distributing devices. The operator sends a boom action instruction via the operating handle, a receiver/emitter (transceiver) receives the instruction and then directly converts the instruction into control signals i1, i2 . . . in for the electrical proportional control valves, and thereby outputs flows Q1, Q2 . . . Qn to the actuating mechanisms respectively; the relation between the control signals i1, i2 . . . in and flows Q1, Q2 . . . Qn depends on the characters of the valves.
In the prior art, when a plurality of knuckle arms move in a composite manner, if the sum of the flows demanded by the oil cylinders (actuating mechanisms) of all knuckle arms is greater than the maximum flow output from the pump of the boom, i.e.:
      Q    S    <            ∑              i        =        1            n        ⁢          Q      i      
The drawback of load-sensitive hydraulic system will be exposed, i.e., the flow is taken to meet the demand of knuckle arms that bear lower loads, while the speeds of movement of knuckle arms that bear higher loads can't reach the values expected by the operator; as a result, the motion track of the boom end will deviate from that expected, and the operator has to change the openness of the operating handle continually to correct the motion track, and thereby the labor intensity is severely increased, and the operating efficiency and movement stability of the boom are compromised.
For example, during the operation of the boom of a concrete pump truck, the load is the lowest in revolving motion; if the knuckle arms revolve and stretch out at the same time, the stretching speed of the boom may be very low or even the boom can't stretch out; consequently, the motion track of the boom end will deviate from that expected.
As shown in FIG. 3, the load of boom revolution is 100 bar, while the load of boom stretching is 200 bar; it is seen from the flow curve of boom revolution and the flow curve of boom stretching: the flow of the hydraulic system is taken to meet the demand of boom revolution first, while only the surplus part of flow is taken to meet the demand of boom stretching.